I'm trying to write a generalization of map that supports multiple return
values:
(define (map-n n f ls . lss)
(if (empty? ls)
(apply values (build-list n (lambda _ '())))
(call-with-values
(thunk (apply f (car ls) (map car lss)))
(lambda vs
(call-with-values
(thunk (apply map-n n f (cdr ls) (map cdr lss)))
(lambda lss
(apply
values
(map cons vs lss))))))))
This version works fine, but, tediously, requires I specify how many return
values.
This is annoying, since surely the context knows how many values it expects:
> (let/ec escape
(let-values ([(a b c)
(let/cc k
(escape (procedure-arity k)))])
'meow))
> (arity-at-least 0)
Oh, I guess not.
Well that's fine, because surely I can reflect on `f`'s result arity:
> (procedure-result-arity (lambda (x) (values 'a 'b 'c)))
> #f
Welp.
So, uh, can I haz a current-context-arity please?
--
William J. Bowman
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